Capacitive membrane ultrasonic transducers have a metal coated membrane such as silicon or silicon nitride supported above a substrate by an insulating layer such as silicon oxide, silicon nitride or other insulating material. The substrate may be a highly doped semiconductor material such as silicon or may be undoped silicon with a metal layer. The thin metal covering the membrane and the highly doped substrate or metal layer form the two electrodes of a capacitor. Generally the substrate, support and membrane form a cell which may be evacuated. Generally the transducers comprise a plurality of cells of the same or different sizes and shapes. In operation, the cells may be arranged in arrays with the electrical excitation generating beam patterns. Typically transducer cells have sizes ranging between 5 μm and 1000 μm in diameter.
The fabrication and operation of capacitive membrane transducers is described in many publications and patents. For example U.S. Pat. Nos. 5,619,476, 5,870,351 and 5,894,452, incorporated herein by reference, describe fabrication using surface machining technologies. Pending Application Ser. No. 60/683,057 filed Aug. 7, 2003, incorporated herein by reference, describes fabrication by using wafer bonding techniques. Such transducers are herein referred to a capacitive micromachined transducers (cMUTS).
The active part of a cMUT is the metal-coated membrane. A DC bias voltage applied between the membrane and the bottom electrodes creates electrostatic attraction, pulling the membrane toward the substrate. If an AC voltage is applied to a biased membrane, harmonic membrane motion is obtained. The DC bias voltage strongly affects the AC vibrational amplitude. As the DC voltage is increased, a larger sinusoidal membrane motion increases the transmitted acoustic pressure. To achieve maximum efficiency, the conventional operation of the cMUT requires a bias voltage close to the collapse voltage, the voltage at which the membrane contacts the substrate. In conventional operation the sum of the DC bias and the applied AC signal must not exceed the collapse voltage. Therefore, total acoustic output pressure is limited by the maximum allowed AC voltage.
If a biased cMUT membrane is subject to an impinging ultrasonic pressure field, the membrane motion generates AC detection currents. This current amplitude increases with increasing DC bias voltage. To maximize the receive sensitivity, the bias voltage is increased close to the collapse voltage. Again, it is required that the sum of the bias voltage and the received voltage due to the motion caused by the ultrasonic pressure field be less than the collapse voltage. In co-pending application Ser. No. 11/078,795 filed Mar. 10, 2005 there is described a method of operating the transducers with the membrane collapsed. In this regime, the membrane is first biased at a voltage higher than the collapse voltage, therefore initially collapsing the membrane onto the substrate. Then, the DC bias is changed to a level, which is larger than the snapback voltage to ensure the collapsed membrane state. At this operating DC voltage, the center of the membrane remains in contact with the substrate. By adding an AC voltage, harmonic membrane motion is obtained in a circular ring concentric to the center. In this regime, the ultrasonic transducer has a higher electromechanical coupling efficiency than it has when it is operated in the conventional pre-collapse regime.
There is nevertheless a need for a method of operating cMUTs to generate higher acoustic output pressures than either the conventional or collapsed method of operation. An ultrasonic transducer which produces high transmit pressures will meet the extreme acoustic transmit pressure demands of the ultrasonic industry.